Highly sensitive accelerometer

ABSTRACT

A highly sensitive accelerometer for determining the acceleration of a structure includes a mass within a housing suspended by opposing support members. The support members are alternately wound around a pair of fixed mandrels and the mass in a push pull arrangement. At least a portion of one of the support members comprises a transducer capable measuring the displacement of the mass within the housing. An embodiment of the invention employs optical fiber coils as support members for use in interferometric sensing processes. Arrays of such interferometer based accelerometers maybe multiplexed using WDM or similar methods.

TECHNICAL FIELD

[0001] This invention relates to highly sensitive accelerometers, andmore particularly to a fiber optic based accelerometer.

BACKGROUND ART

[0002] It is known to monitor the physical characteristics of structuresand bodies using sensors. One such application is the monitoring of oilwells to extract such information as temperature, pressure, fluid flow,seismic and other physical characteristics. The monitoring of oil wellspresents certain challenges for conventional sensors because of theharsh environment in terms of high pressures and temperatures and hashistorically been dominated by the use of electronic sensors and opticalsensors to a lesser degree.

[0003] The presently used electrical sensors are limited for severalreasons. The on-board electronics of such sensors must operate in a veryhostile environment, which includes high temperature, high vibration andhigh levels of external hydrostatic pressure. Such electrical sensorsalso must be extremely reliable, since early failure entails a very timeconsuming and expensive well intervention. Electronics, with itsinherent complexity, are prone to many different modes of failure. Suchfailures have traditionally caused a less than acceptable level ofreliability when these electrical sensors are used to monitor oil wells.

[0004] There are numerous other problems associated with thetransmission of electrical signals within well bores. In general, thereare many problems encountered in providing an insulated electricalconductor for transmitting electrical signals within well bores. Suchelectrical conductors are extremely difficult to seal against exposureto well bore fluids, which are at high temperatures, high pressures, andpresent a very corrosive environment. Such electrical conductors, oncedamaged by the fluids that penetrate the insulating materials around theelectrical conductors, will typically short electrical signals.Additionally, electrical transmissions are subject to electrical noisesin some production operations.

[0005] It is typical to use an accelerometer to measure downhole seismicdisturbances to determine the acoustic wave characteristics ofunderground layers in proximity of the well bore. In general, it istypical to consider an accelerometer as a mass-spring transducer housedin a sensor case with the sensor case coupled to a moving body, theearth, whose motion is inferred from the relative motion between themass and the sensor case. Such accelerometers are analyzed byconsidering the relative displacement of the mass as being directlyrelated to the acceleration of the case and therefore the earth inproximity of the well bore. An array of accelerometers is typicallyplaced along the length of a well bore to determine a time dependantseismic profile.

[0006] One prior art accelerometer is a piezoelectric based electronicaccelerometer. The piezoelectric based electronic accelerometertypically suffers from the above identified problems common toelectrically based sensors. In particular, most higher performancepiezoelectric accelerometers require power at the sensor head. Also,multiplexing of a large number of sensors is not only cumbersome buttends to occur at significant increase in weight and volume of anaccelerometer array, as well as a decrease in reliability.

[0007] It is also known to use optical interferometers for themeasurement of acceleration of certain structures. It is also well knownthat fiber optic interferometric accelerometers can be designed withfairly high responsivities and reasonably low threshold detectability.Some prior art types of fiber optic accelerometers includeinterferometric fiber optic accelerometers based on linear and nonlineartransduction mechanism, circular flexible disks, rubber mandrels andliquid-filled-mandrels. Some of these fiber optic accelerometers havedisplayed very high acceleration sensitivity (up to 10⁴ radians/g), buttend to utilize a sensor design that is impractical for manyapplications. For instance, sensors with a very high accelerationsensitivity typically tend to have a seismic mass greater than 500 gramswhich seriously limits the frequency range in which the device may beoperated as an accelerometer and are so bulky that their weight and sizerenders them useless in many applications. Other fiber opticaccelerometers suffer either from high cross-axis sensitivity or lowresonant frequency or require an ac dither signal and tend to be bulky(>10 kg), expensive and require extensive wiring and electronics. Evenoptical interferometers designed of special material or construction aresubject to inaccuracies because of the harsh borehole environment andbecause of the very tight tolerances in such precision equipment.

[0008] For many applications, the fiber optic sensor is expected to havea flat frequency response up to several kHz (i.e., the device must havehigh resonant frequency), high sensitivity, must be immune to extraneousmeasurands (e.g., dynamic pressure), must have a small foot print andpackaged volume that is easily configured in an array (i.e., easymultiplexing).

SUMMARY OF THE INVENTION

[0009] Objects of the present invention include provision of a fiberoptic accelerometer for use within a harsh environment.

[0010] The invention may be used in harsh environments (hightemperature, and/or pressure, and/or shock, and/or vibration), such asin oil and/or gas wells, engines, combustion chambers, etc. In oneembodiment, the invention may be an all glass fiber optic sensor capableof operating at high pressures (>15 kpsi) and high temperatures (>150°C.). The invention will also work equally well in other applicationsindependent of the type of environment.

[0011] It is an object of the present invention to provide a highlysensitive linear accelerometer for sensing acceleration in apredetermined direction. The accelerometer is comprised of a rigidhousing a mass suspended therein by at least two elastic support memberswhich are axially aligned in the predetermined direction and attached toopposite ends of the housing and further attached to the mass. At leasta portion of one of the elastic support members comprises a transducercapable of measuring a displacement of the mass within the housing inresponse to an acceleration along the predetermined direction. Certainembodiments include a pair of fixed mandrels rigidly attached toopposite ends of the housing and the mass comprises at least onefloating mandrel where the elastic support members are each wrappedaround one of the fixed mandrels and the floating mandrel.

[0012] It is another object of the present invention to provide a linearaccelerometer where the mass comprises a pair of floating mandrels andwherein each elastic support member is wrapped about one of the fixedmandrels and one the floating mandrels. In another embodiment themandrels and the mass of the accelerometer comprise a toroidal shape.

[0013] It is yet another object of the present invention to provide alinear accelerometer where at least one of the elastic support memberscomprises an optical fiber the movement of the mass induces in the fibera variation in length corresponding to the movement for interferometricmeasurement of the variation in length of the fiber.

[0014] It is still another object of the present invention to provide alinear accelerometer having an axial alignment assembly attached to themass limiting movement of the mass in a direction perpendicular to thepredetermined direction where the axial alignment assembly comprises aflexure member attached to the mass and the housing allowing axialmovement of the mass in the predetermined direction and limits non-axialmovement of the mass. In an embodiment a pair of alignment assembliesare employed where the flexure member is a diaphragm positioned on analignment rod and the diaphragm is captured within a bore in the housingabout their outer periphery. Another embodiment provides for a borepositioned in the fixed mandrels for capturing the diaphragms. Inanother embodiment the flexure member comprises a thin flexible plateand at least one pair of the flexure members are attached to the massand to the housing.

[0015] It is still further an object of the present invention to providethe linear accelerometer where the transducer comprises a strain sensingelement including a fiber optic strain sensor, a piezo electric device,a PVDF material or a resistive strain gage.

[0016] It is another object of the presenting invention to provide ahighly sensitive linear accelerometer for sensing acceleration in apredetermined direction having a rigid housing and a mass having anelongated body and rounded ends, and a pair of fixed mandrels rigidlyattached to the housing a predetermined distance apart, and two pairs ofelastic support members axially aligned in the predetermined directionand wrapped around the fixed mandrels and the rounded ends in acontinuous fashion to suspend the mass within the housing. At least aportion of one of the elastic support members comprises a transducercapable of measuring a displacement of the mass within the housing inresponse to an acceleration along the predetermined direction furtherincludes a pair of axial alignment assemblies attached to the masslimiting movement of the mass in a direction perpendicular to thepredetermined direction.

[0017] It is yet another object to provide a linear accelerometer wherethe fixed mandrels and the mass are comprised of a toroidal shape.

[0018] It is still another object of the present invention to provide anapparatus for vertical seismic profiling of an earth borehole having anx-direction, a y-direction and a z-direction orthogonal to each other,where the apparatus includes an optical fiber transmission cable and aplurality accelerometers coupled to the borehole and in opticalcommunication with the optical fiber transmission cable and positionedin each of the three orthogonal directions. The accelerometer is ahighly sensitive linear accelerometer for sensing acceleration in apredetermined one of the directions, the accelerometer includes a rigidhousing, a mass, at least two elastic support members comprised ofoptical fiber axially aligned in the predetermined direction andattached to opposite ends of the housing and further attached to themass, the elastic support members suspending the mass within the housingwherein at least a portion of one of the elastic support memberscomprises a transducer capable of measuring a displacement of the masswithin the housing in response to an acceleration along thepredetermined direction and providing a respective sensing light signalindicative of static and dynamic forces at a respective accelerometerlocation. The apparatus also includes an optical signal processorconnected to the optical transmission cable providing seismic profileinformation based on the respective sensing light signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic diagram of a acceleration monitoring systemincorporating a highly sensitive accelerometer in accordance with thepresent invention;

[0020]FIG. 2 is a cross-sectional view of an earth borehole having anarray of accelerometers of the invention deployed therein for verticalseismic profiling;

[0021]FIG. 3 is a schematic diagram of a spring mass acceleration modelof the prior art;

[0022]FIG. 4 is a side view of a schematic representation of anaccelerometer of in accordance with the present invention;

[0023]FIG. 5 is a top view of a schematic representation of anaccelerometer of in accordance with the present invention;

[0024]FIG. 6 is a perspective view of an embodiment of the accelerometerof the present invention;

[0025]FIG. 7 is an exploded perspective view of the accelerometer ofFIG. 6 showing the axial alignment assemblies;

[0026]FIG. 8 is a perspective view of an embodiment of the accelerometerof the present invention;

[0027]FIG. 9 is a perspective view of the mass and axial alignmentassemblies of the accelerometer of FIG. 8;

[0028]FIG. 10 is a perspective view of an embodiment of theaccelerometer of the present invention comprised of toroidal shapedmembers;

[0029]FIG. 11 is a perspective view of an embodiment of theaccelerometer of FIG. 6 having an alternative axial alignment assembly;

[0030]FIG. 12 is a graphical representation of the response of anembodiment of the present invention to a test signal;

[0031]FIG. 13 is a graphical representation of the phase response of theembodiment of FIG. 12;

[0032]FIG. 14 is a graphical representation of the amplitude response ofthe embodiment of FIG. 12;

[0033]FIG. 15 is a side view of an elastic support member comprising anoptical fiber wrap having a pair of Bragg gratings around each opticalwrap, in accordance with the present invention;

[0034]FIG. 16 is a side view of optical fiber wrap with a pair Bragggratings within each wrap, in accordance with the present invention;

[0035]FIG. 17 is a side view of optical fiber wrap interferometer, inaccordance with the present invention;

[0036]FIG. 18 is a top view in partial section of an elastic supportmember having an optical fiber with a pair of Bragg gratings, inaccordance with the present invention;

[0037]FIG. 19 is a top view in partial section of an alternativegeometry of an elastic support member having an optical fiber with apair of Bragg gratings, in accordance with the present invention;

[0038]FIG. 20 is a top view in partial section of an elastic supportmember having an alternative geometry optical fiber in the form of aradiator coil;

[0039]FIG. 21 is a top view in partial section of an elastic supportmember having an alternative geometry optical fiber in the form of arace track;

[0040]FIG. 22 is a top view of three alternative strain gauges, inaccordance with the present invention; and

[0041]FIG. 23 is a top view in partial section of an elastic supportmember showing a strain gage.

BEST MODE FOR CARRYING OUT THE INVENTION

[0042] Referring to FIG. 1, a structure 10 subjected to a hostileenvironment, such as an oil or gas well borehole, building, bridge,aircraft, pump or other structure or component subjected to accelerationand wishing to be interrogated has coupled to it at least one highlysensitive accelerometer 22 as will be more fully described herein below.Highly sensitive accelerometer 22 is part of transmission cable string20 connected by transmission cable 28 to a signal converter 40 andsignal processing equipment 35. The acceleration of structure 10 in anyof the three axes 30, 32, 34 is detected by accelerometer 22, dependingon the orientation of the accelerometer, as will be more fully describedherein after. The signal processing equipment may comprise any knowninstrumentation for processing the electrical, electro-optic, or opticalsignal of the various embodiments of the present invention.

[0043] In a particular embodiment of the present invention,accelerometer 22 is mounted within a hermitically sealed vessel (notshown) and is disposed in a harsh environment having a high temperature(up to about 175 degrees C.), high pressure (up to about 20 ksi), a highEMI environment or any non-harsh environment where a highly sensitiveaccelerometer is needed. Also in certain embodiments, accelerometer 22may comprise a fiber optic based device and transmission cable 28 maycomprise an environmentally hardened capillary tube such as thatdisclosed in commonly owned, copending U.S. patent application Ser. No.09/121,468, title Optical Fiber Cable for Use in Harsh Environments, toBonja, filed Jul. 23, 1998, the disclosure of which is incorporatedherein in its entirety. The transmission cable 28 is routed toaccelerometer 22 and provides for the delivery of communication signalsbetween the signal processing equipment 35 and is connected therebetweeneither directly or via interface equipment (not shown) as required. Theaccelerometer is closely coupled to the structure by bolting, clampingor other known methods.

[0044] Accelerometer 22 of the present invention may be used, forexample, as a single device to monitor structure 10 directly, in anarray of similar such accelerometers to monitor structure 10. In onealternative an array of accelerometers 22 may be coupled to a structure10 to determine the structure's response to the surrounding environmentsuch as, for example for performing vertical seismic profiling, and asare distributed over a known length. Referring to FIG. 2, structure 10maybe any structure, such as a casing or production pipe, coupled to aborehole within an oil or gas well, and penetrates various earth layers12, 14, 16. Such a borehole may be fifteen to twenty thousand feet ormore in depth. As is known in the art, the borehole is filled with ahigh temperature and pressure drilling fluid 18 which presents anextremely corrosive and hostile environment. Transmission string 20includes an array of accelerometers 22, 23, 24, 25 as described hereinabove connected by transmission cable 28 which may comprise an opticalfiber positioned within a capillary tube. The accelerometers 22, 23, 24,25 may comprise a single accelerometer or may comprise two or threelinear accelerometers 22 of the present invention positioned in any ofthe three axes 30, 32, 34 (FIG. 1) and transmit a respective sensinglight signal indicative of static and dynamic forces at the respectiveaccelerometer location.

[0045] The array of accelerometers 22, 23, 24, 25 is useful forperforming the vertical seismic profiling of the invention, with theoptical fiber sensors distributed over a known length, such as 5000feet. Over the known length, the accelerometers 22, 23, 24, 25 areevenly spaced at a desired interval, such as every 10 to 20 feet, forproviding the desired vertical seismic profiling. As described ingreater detail herein, each accelerometer includes fiber optic sensorsthat reflect a narrow wavelength band of light having a centralwavelength. Each accelerometer operates at a different wavelength bandand central wavelength such that the signals may be easily detectedusing Wavelength Division Multiplexing (WDM) techniques. The entireoptical fiber, positioned within the transmission cable 28, is loweredto a desired depth, for example as measured from the upper most sensor,such as 1,000 feet. An acoustic wave source, such as a small charge ofdynamite 42 (a seismic shot), is detonated by a blaster 45 in a shallowshothole 50 that is offset from the borehole 10 by a selected distance,such as 3,000 feet.

[0046] Still referring to FIG. 2, acoustic waves radiate from the shotalong a direct path 52 and a reflected path 54. The reflected waves 54are reflected off of the various earth layers 12, 14, 16. As will bedescribed in greater detail hereinafter, the direct seismic waves 52 andreflected seismic waves 54 cause the surrounding various earth layers12, 14, 16 to react and the motion of the earth is detected by theaccelerometers 22, 23, 24, 25 through structure 10 coupled to the earth.Resulting data signals are transmitted through the transmission cable 28to the demodulator 40 and optical signal processing equipment 35. In oneembodiment of the invention, after the seismic shot, the transmissioncable string 20 is repositioned within the borehole for additionalseismic profiling. In another embodiment of the invention, theaccelerometers 22, 23, 24, 25 are distributed over the entire length ofthe transmission cable 28 such that the entire borehole 10 ischaracterized in a single shot.

[0047] In an array of accelerometers of the present invention, eachaccelerometer operates at a different wavelength band and centralwavelength such that the signals may be easily detected using WavelengthDivision Multiplexing (WDM) techniques. Signal processing equipment 35and signal converter 40, which may comprise one or more demodulators,interpret the wavelength phase change from the return signals.

[0048] A number of prior art performance deficiencies are addressed byaccelerometer 22 in accordance with the present invention. For instance,for fiber optic based embodiments, the lowest resolvable or measurableacceleration will be limited by the detection noise floor of theinterferometer which is configured around the optical fiber coils 80, 82(FIG. 4), in conjunction with the phase measurement scheme and the scalefactor of the accelerometer mechanism. In, for instance, seismicapplications, though the present invention is not limited to such,accelerometer 22 is required to detect accelerations as low as 10-100G/rtHz. Furthermore, it is well known that high performanceinterferometers and phase measurement systems can detect phase shifts aslow as 10 to 100 microad/rtHz or even better. The optical fiber coils ofthe support members of an interferometer with an associated phasemeasurement system, yield an accelerometer sensitivity or scale factorof about 1 krad/G, or higher, to achieve measurements with the indicatednoise floor (FIG. 12 is an example of a typical test signal relative tothe noise floor of an embodiment of the present invention).

[0049] With reference to FIG. 4, accelerometer 22 maybe fabricated withscale factors of between 500 and 5000 krad/G, that covers the range ofscale factors, as detailed herein below, necessary to use thisaccelerometer in seismic applications. As previously noted,interferometer measurement systems exhibit scale factors that increasewith increasing fiber length. The fixed 86, 88 and floating mandrels 90,92 are used to create multiple coil turns of fiber 66 in each supportmember, thereby enabling a small package for an accelerometer with highscale factor. In this accelerometer 22, the effective scale factor canbe described in terms of the strain applied to the fiber by the movingmass of the floating mandrels. It is interesting to note that the scalefactor will be proportional to the mass of the design and inverselyproportional to the cross sectional area of the supporting fiber.Normally, as the length of the fiber of an interferometer 62 (FIG. 3)increases, the sensitivity increases. However, the supporting fiberconsists of a number of turns in the suspension coil. As the fiberlength increases, the number of turns increases and the total fibercross sectional area of the suspension bands increases. The effect is tomake the scale factor approximately independent of total fiber length.

[0050] The range of accelerometer 22 can be limited by one of 2 factors.For instance, if the phase measurement system has a limited range, thenlarge accelerations cannot be interpreted. However, current phasedemodulator technology, as typified by an Optiphase model OPD-200,produced and sold by Optiphase, that can track phase changes over many2π cycles, for example, removes this aspect as a limitation.

[0051] The other potential limitation might be the mechanical strengthof the fiber. The present invention has been reviewed with respect tothe mechanical implications of large acceleration changes imposed on thesuspension coils. It is useful to realize that even at very high shockconditions, for example as high as 200 G's, that the transient load isshared by all of the fibers in the coil. In such a situation, themaximum load applied to any filament in the coil can be much less than10% of the ultimate strength of the glass filament. This load sharingability is a benefit of the accelerometer of the present inventiondemonstrating inherent durability and large acceleration rangecapability.

[0052] A typical approach for accelerometer design is to define theoperating bandwidth to be the flat signal response spectral region belowthe first structural resonance of the suspended mass. In the case ofaccelerometer 22, it is important to keep in mind that the stiffness ofthe coils will has an impact on the resonant frequency and the totalglass cross sectional area of the coil relative to the accelerometermass must be considered when designing the fundamental resonantfrequency. We have discovered that an adequate scale factor can beachieved while maintaining the system resonance above 1 kHz. Thisdiscovery enables the present invention to satisfy many seismictransducer application requirements. Examples of both the amplitude andphase response functions of a typical device are shown in FIGS. 13 and14, verifying the ability to achieve high resonant frequencies whileachieving good sensitivity. Certain embodiments of accelerometer 22 makeit practically insensitive to position with respect to gravity as willbe shown in greater detail herein below.

[0053] In practice it is generally not practical to use long fiberlength l in a single strand as shown in FIG. 3. As such the presentinvention uses multiple windings 80, 82 of fiber 66 to obtain a longeffective fiber length as best shown with reference to FIGS. 3 and 4.The windings 80, 82 of fiber optic accelerometer 22 each comprise Nturns of fiber 66 coiled around a fixed mandrel 86, 88 and around asecond active mandrel 90, 92 that is free and used to strain the fiberby its own mass. The fixed mandrels may be grounded to a housingrepresented by 98 and the active mandrels may be restrained frommovement normal to the axial direction represented by arrow 70. Whenhousing 98 is subjected to motion in the axial direction 70 theacceleration associated with that motion is detected by the transducers,or sensor coils 94, 96 in a manner similar to the mass/spring system ofFIG. 3. A single sensor coil 94 or 96, could be used to measureacceleration in the axial direction, however the push-pull ordifferential arrangement of the pair of sensor coils (in aninterferometer for example) 94, 96 of fiber optic accelerometer 22provides mechanical symmetry which lowers total harmonic distortion andcross axis sensitivity. Mechanical symmetry could also be achieved byreplacing one of the sensor coils with another material having asubstantially similar spring rate. In essence the active mandrels 90, 92are suspended between at least one pair of springs or elastic supportmembers, at least a portion of one of which is a strain sensing element,preferably comprised of optical fibers 66.

[0054] In alternative embodiments, one of the pair of sensor coils 94,96 may be either be used as a dummy arrangement to create mechanicalsymmetry in the axial direction 70, or as a back-up arrangement in theevent that one of the sensor coils fails, or as a secondary sensor coilin a push-pull or differential arrangement. The latter effectivelydoubles the accelerometer scale factor.

[0055] Any known optical fiber may be used having various diameters,however the fiber diameter is important to the performance as well asthe durability and reliability of the accelerometer. For example, anoptical fiber having a relatively large diameter has a minimum bendradius to ensure a predictable lifetime with failure. If a largediameter fiber is used, a commensurately large mandrel diameter shouldbe used to accommodate the fiber for reliability reasons. However, asmandrel diameter grows so too does the overall volume of accelerometer22.

[0056] Referring to FIG. 6 there is shown an embodiment of accelerometer22 in accordance with the present invention as described herein aboveincluding 3 elastic support members 150, 152, 154 comprised of windingsof optical fibers, although other elastic support members could beemployed without deviating from the present invention. Elastic supportmembers 150, 152 combined are comprised of the same length of fiber aselastic support member 154 and cooperate in a push-pull arrangement tosuspend mass 156 within housing 158. The wraps of support 154 are woundin a continuous fashion about fixed mandrel 160 rigidly attached tohousing 158 and mandrel end 162 of mass 156. Similarly the wraps ofsupport members 150, 152 are wound in a continuous fashion about fixedmandrel 164 rigidly attached to housing 158 and mandrel end 166 of mass156. The support member 150, together with support member 152, compriseone sensor coil and support member 154 comprises a second sensor coil,both being similar to sensor coil 94, 96 described with reference toFIGS. 3 and 4. Support members 150, 152 act as a spring to bias knownproof mass 156 against the spring action of support member 154 andcooperate to suspend the mass within housing 158. The fixed mandrels160, 162 are positioned within the housing 158 to produce apredetermined initial bias in each of the elastic support members 150,152, 154. Support members 150, 152, 154 are axially aligned, in thedirection indicated by arrow 168, with each other, housing 158 and mass156. Fixed mandrels 160, 164 and mandrel ends 162, 166 include grooves170 positioned thereon to facilitate assembly and maintain the axialpositioning of the support members. Accelerometer 22 accurately detectsacceleration in the axial direction 168 as will be more fully explainedherein below.

[0057] Mass 156 is comprised of central portion 171 between mandrel ends162, 166, however embodiments of the present invention include thosewherein a single cylindrical floating mandrel comprises the total massand around which both interferometers are wound. Mass 156 ofaccelerometer 22 further includes alignment assemblies 172, 174 as bestshown in FIGS. 6 and 7 for limiting the movement of mass 156perpendicular to the axial direction 168. Alignment assemblies 172, 174are comprised of alignment rods 176, 178 that slidably pass throughholes 180, 182 in mandrel ends 162, 166 respectively and are attached todiaphragms 184, 186 by, for example, threaded nuts 188, 190. Diaphragms184, 186 are captured within bores 192, 194 in housing 158 by end plates196, 198 installed on the ends of the housing by screws (not shown) forexample. Boss 200 on end plates 196, 198 cooperate with lip 202 withinbores 192, 194 to capture diaphragms 184, 186 about their outer edgeswithin the bore to allow for flexure of the diaphragms in the axialdirection 168. Diaphragms 184, 186 are comprised of a thin flexiblematerial, metal for example, which provides for a highly flexible memberalong the axial direction 168 but is quite rigid in the plane of thediaphragms (perpendicular to the axial direction). This allowsrelatively unimpeded movement of mass 156 in the axial direction 168while virtually eliminating movement of the mass assembly in non-axialdirections. By limiting the movement of the mass 156 in non-axialdirections, alignment assemblies 172, 174 of accelerometer 22 greatlyreduce cross-axis response. Alternative embodiments of the alignmentassemblies include ones wherein the holes 180, 182 cooperate withalignment rods 176, 178 in a close tolerance arrangement to preclude theneed for diaphragms 184, 186. In this particular embodiment thealignment rods 176, 178 limit movement of the mass 156 in non-axialdirection by interference with the walls of the holes 180, 182.

[0058] In operation accelerometer 22 may be mounted to a structure, suchas an oil production tube 10 (FIG. 2) for example, by rigid attachmentof housing 158 by any method such as bolting, welding or other knownmethods. As the structure experiences acceleration due to changes inmovement direction, or relative velocity, mass 156 will shift in theaxial direction 168 within housing 158 in a magnitude related to theacceleration of the structure in the axial direction. Elastic supportmembers 150, 152 154 will respond by elongating or relaxing, the actionof which will lengthen or shorten the optical fibers and produce asignal corresponding to the acceleration. For example, when thestructure, or housing 158 thereby, is accelerated in the directionindicated by arrow 210 mass 156 will be displaced within the housing inthe opposite direction indicated by arrow 211. In this particular casethe tension in support member 154 will increase (and the fiber lengththerein will therefore increase) and the tension in support members 150,152 will decrease (and the fiber length therein will decrease).Similarly, when the structure, or housing 158 thereby, is accelerated inthe direction indicated by arrow 211 mass 156 will displaced within thehousing in the opposite direction indicated by arrow 210. In thisparticular case the tension in support members 150, 152 will increase(and the fiber length therein will therefore increase) and the tensionin support member 154 will decrease (and the fiber length therein willdecrease). The change in phase angle of the light within the fibers asinterpreted by the processing equipment 35 (FIG. 1) caused by the changein length of the fibers corresponds to a known acceleration level asdescribed herein above. The support members are independent coil systemsand their output can be manipulated accordingly in known a manner suchas differential or in an independent mode as a single coil in a sensorleg of an interferometer. Other methods of determining a correspondingchange in length of the support members are included in the presentinvention and will be more fully described herein below. In analternative embodiment only one of the interferometers, either the onecomprised by support member 154 or the one comprised by support members150, 152 is used for outputting a signal responsive to the accelerationof the accelerometer 22.

[0059] Referring to FIGS. 8 and 9 there is shown another embodiment ofaccelerometer 22 in accordance with the present invention as describedherein above including two pairs of elastic support members 150, 152,154, 155 comprised of windings of optical fibers, although other elasticsupport members could be employed without deviating from the presentinvention. Elastic support members 150, 152 are comprised of the samelength of fiber as elastic support members 154, 155 and cooperate in apush-pull arrangement to suspend mass 156 within housing 158. The wrapsof supports 154,155 are wound in a continuous fashion about fixedmandrel 160 rigidly attached to housing 158 and mandrel end 162 of mass156. Similarly the wraps of support members 150, 152 are wound in acontinuous fashion about fixed mandrel 164 rigidly attached to housing158 and mandrel end 166 of mass 156. Each of the support members 150,152, 154, 155 comprise a sensor coil for use in an interferometer, withall being similar to sensor coils 94, 96 described with reference toFIGS. 3 and 4. Support members 150, 152 act as a spring to bias mass 156against the spring action of support members 154, 155 and cooperate tosuspend the mass within housing 158. The fixed mandrels 160, 162 areinitially positioned within the housing 158 to produce a predeterminedinitial bias in each of the elastic support members 150, 152, 154, 155.Support members 150, 152, 154, 155 are axially aligned, in the directionindicated by arrow 168, with each other, housing 158 and mass 156. Fixedmandrels 160, 164 and mandrel ends 162, 166 include grooves 170positioned thereon to facilitate assembly and maintain the axialpositioning of the support members. Accelerometer 22 accurately detectsacceleration in the axial direction 168 as will be more fully explainedherein below.

[0060] Accelerometer 22 as shown in FIG. 8 is small enough to fit withina tube 91, having end caps 93 for use in sealing and protecting thedevice from the environment. Tube 91, in one embodiment, is comprised ofInconel material and has outside dimensions of approximately one inch indiameter and approximately 3.5 inches in length. At least one of endcaps 93 further includes an exit hole 97 including any known sealingfeature for routing the transmission cable 28 (FIG. 1) from the housing.The mandrel diameters 100 are approximately 11-13 mm and the distancebetween fixed mandrels 160, 164 and floating mandrels 162, 166respectively is about 44 mm in a 0.0 g state. Mass 156 is comprised of ametallic material and is approximately 60 grams. Support members 150,152, 154, 155 are comprised of an 80 micron optical fiber and a totallength of between about 10 m and about 20 m is used with the number ofwraps varying from about 39 to about 105. The housing 158, mass 156 andmandrels are may all be comprised of metal materials. In embodimentswhere the support members are comprised of optical fibers, the use of anall metal configuration with glass fibers yields an extremely stable andreliable accelerometer 22 even at elevated temperatures.

[0061] Referring to FIG. 9, mass 156 is includes cylindrically shapedmandrel ends 162, 166, however embodiments of the present inventioninclude those wherein a single cylindrical floating mandrel comprisesthe total mass and around which both sensor coils are wound. Mass 156 ofaccelerometer 22 further includes alignment assemblies 172, 174 forlimiting the movement of mass 156 perpendicular to the axial direction168. Alignment assemblies 172, 174 are comprised of alignment rods 176,178 respectively and are attached to diaphragms 184, 186 by, forexample, welding or gluing. Diaphragms 184, 186 are captured withinbores 192, 194 (not shown) in housing 158 about their outer edges withinthe bore to allow for flexure of the diaphragms in the axial direction168. Diaphragms 184, 186 are comprised of a thin flexible material,metal for example, which provides for a highly flexible member along theaxial direction 168 but is quite rigid in the plane of the diaphragms(perpendicular to the axial direction). This allows relatively unimpededmovement of mass 156 in the axial direction 168 while virtuallyeliminating movement of the mass assembly in non-axial directions. Bylimiting the movement of the mass 156 in non-axial directions, alignmentassemblies 172, 174 of accelerometer 22 greatly reduce cross-axisresponse.

[0062] In operation accelerometer 22 may be mounted to a structure, suchas an oil well casing, or an oil production tube 10 (FIG. 2) forexample, by rigid attachment of housing 158 by any method such asbolting, welding or other known methods. As the structure experiencesacceleration due to changes in movement direction, or relative velocity,mass 156 will shift in the axial direction 168 within housing 158 in amagnitude related to the acceleration of the structure in the axialdirection. Elastic support members 150, 152 154, 155 will respond byelongating or relaxing, the action of which will lengthen or shorten theoptical fibers and produce a signal corresponding to the acceleration.For example, when the structure, or housing 158 thereby, is acceleratedin the direction indicated by arrow 210, mass 156 will be displacedwithin the housing in the opposite direction indicated by arrow 211. Inthis particular case the tension in support members 154, 156 willincrease (and the fiber length therein will therefore increase) and thetension in support members 150, 152 will decrease (and the fiber lengththerein will decrease). Similarly, when the structure, or housing 158thereby, is accelerated in the direction indicated by arrow 211, mass156 will displaced within the housing in the opposite directionindicated by arrow 210. In this particular case the tension in supportmembers 150, 152 will increase (and the fiber length therein willtherefore increase) and the tension in support members 154, 155 willdecrease (and the fiber length therein will decrease).

[0063] Referring to FIG. 10 there is shown yet another embodiment of thepresent invention wherein fixed mandrels 160, 164 are both in the formof a torus having an internal bore 161, 163 and wherein mass 156 is inthe form of an elongated torus having a bore 165. Fixed mandrels areattached to a housing represented by 158 and similar to that describedhereinabove by any known method. In accordance with the presentinvention and as described herein above four pairs of elastic supportmembers 150, 151, 152, 153 bias mass 156 toward fixed mandrel 160 andfour pairs of elastic support members 154, 155, 157, 159 bias mass 156toward fixed mandrel 164. Although the embodiment in FIG. 10 is shownwith reference to four pairs of supports members, the present inventionincludes more pairs. Also, although shown as a torus, the mass and fixedmandrels may comprise any shape which permits placement of supportmembers in a 360 degree distributed fashion about the mandrels and mass.Elastic support members 150, 151, 152, 153 are comprised of the samelength of fiber as elastic support members 154, 155, 157, 159 andcooperate in a push-pull arrangement to suspend mass 156 within housing158. The wraps of supports 154, 155, 157, 159 are wound in a continuousfashion about fixed mandrel 160 through bore 161 rigidly attached tohousing 158 and through bore 165 of mass 156 around and mandrel end 162.Similarly the wraps of support members 154, 155, 157, 159 are wound in acontinuous fashion about fixed mandrel 164 through bore 163 rigidlyattached to housing 158 and through bore 165 and mandrel end 166 of mass156. Each of the support members may comprise a coil for use in aninterferometer, with all being similar to sensor coils 94, 96 describedwith reference to FIGS. 3 and 4. Support members 150, 151, 152, 153 actas a spring to bias mass 156 against the spring action of supportmembers 154, 155, 157, 159 and cooperate to suspend the mass withinhousing 158. The fixed mandrels 160, 162 are initially positioned withinthe housing 158 to produce a predetermined initial bias in each of theelastic support members. Support members 150-159 are axially aligned, inthe direction indicated by arrow 168, with each other, housing 158 andmass 156 and are preferably evenly distributed in the radial direction.

[0064] Referring to FIG. 11 there is shown an alternative embodiment ofaxial alignment assemblies 172, 174 comprised of flexure members 184-187are attached to the mass 156 and the housing 158 near their outboardends by, for example, welding or gluing, to allow for flexure of theflexure members in the axial direction 168. Flexure members 184-187 arecomprised of a thin flexible material, metal for example, which providesfor a highly flexible member along the axial direction 168 but is quiterigid in the plane of the flexure members (perpendicular to the axialdirection). This allows relatively unimpeded movement of mass 156 in theaxial direction 168 while virtually eliminating movement of the massassembly in non-axial directions. By limiting the movement of the mass156 in non-axial directions, alignment assemblies 172, 174 ofaccelerometer 22 greatly reduce cross-axis response.

[0065] The performance of the accelerometer 22 of the present inventionis best shown with reference to FIG. 12 that shows a plot of therelative response of the accelerometer of FIG. 8 to an excitation forceon a calibration test shaker. The test shaker set up is known in theindustry and is comprised of standard input and output components aswell as a known reference accelerometer. The specific accelerometer 22was designed to operate with a bandwidth from about 5 Hz up to about 500Hz. Accelerometer 22 of the present invention was subjected to a testsignal of approximately 126 μg in the axial direction 168 at a frequencyof 25 Hz. Line 101 represents the performance of accelerometer 22 whenthe axial direction was parallel to the z-axis as represented by arrow34 (FIG. 1) and shows an extremely sensitive 65 dB signal to noise ratioresponse represented by point 104 at the 25 Hz test signal frequencywith very little spurious response on either side of the test signal.Similarly line 103 represents the performance of accelerometer 22 whenthe axial direction and the test force are parallel to the x-axis asrepresented by arrow 30 (FIG. 1) and the same 25 Hz test signal. Line103 shows an almost exact level of response at the test signal frequencyof 25 Hz. In addition, the orientation of the accelerometer did notadversely affect the relatively low spurious signals on either side ofthe test signal. The relatively low noise is further demonstrated in thefigure with the largest such peak being less than 28 dB at 60 Hz. The 60Hz signal is due to ground loops in the calibration system and is notconsidered an accelerometer error signal. Such signals, once their causeis identified, can in most instances be isolated and eliminated. It isan important feature of the present invention that orientation of theaccelerometer with respect to gravity has little effect on itsperformance. Therefore, arrays of accelerometers 22 in the threeorthogonal directions 30, 32, 34 (FIG. 2) can be used to measure thevector directions of seismic detected waves.

[0066] Referring to FIGS. 13 and 14 the bandwidth of the accelerometeris shown. The accelerometer was tested as described herein above withreference to FIG. 12 and the phase response was checked against thereference accelerometer (FIG. 13) and the amplitude response was checkedrelative to the reference accelerometer for a frequency range up toabout 500 Hz. The phase response represented by line 105 in FIG. 13 isrelatively flat which demonstrates that the accelerometer 22 isoperating well away from the resonant frequency of the device. Line 105further shows the accelerometer lacks spurious signals within thebandwidth that could otherwise result in errors within the desiredoperating bandwidth. Likewise, the relative amplitude responserepresented by line 107 in FIG. 14 is relatively flat and free ofspurious signals. This further demonstrates that the accelerometer 22 isoperating well away from the resonant frequency of the device andbehaves predictably in the frequency range of 5 Hz to 500 Hz.

[0067] In an embodiment of the present invention that utilizes fiberoptics as the elastic support members, they may be connectedindividually or may be multiplexed along one or more optical fibersusing wavelength division multiplexing (WDM), time division multiplexing(TDM), or any other optical multiplexing techniques (discussed morehereinafter).

[0068] Referring to FIG. 15 the support member comprising a wrap 302,may have a pair of gratings 310, 312 on opposite ends of the wrap 302.The wrap 302 with the gratings 310, 312 may be configured in numerousknown ways to precisely measure the fiber length L or change in fiberlength ΔL, such as an interferometric, Fabry Perot, time-of-flight, orother known arrangements. An example of a Fabry Perot technique isdescribed in U.S. Pat. No. 4,950,883 “Fiber Optic Sensor ArrangementHaving Reflective Gratings Responsive to Particular Wavelengths”, toGlenn. One example of time-of-flight (or Time-Division-Multiplexing;TDM) would be where an optical pulse having a wavelength is launcheddown the fiber 66 and a series of optical pulses are reflected backalong the fiber 66. The length of each wrap can, at any point in time,then be determined by the time delay between each return pulse and therelated acceleration of the mass 156 (FIG. 8) thereby.

[0069] Alternatively, a portion or all of the fiber between the gratings(or including the gratings, or the entire fiber, if desired) may bedoped with a rare earth dopant (such as erbium) to create a tunablefiber laser, such as is described in U.S. Pat. No. 5,317,576,“Continuously Tunable Single Mode Rare-Earth Doped Laser Arrangement”,to Ball et al or U.S. Pat. No. 5,513,913, “Active Multipoint Fiber LaserSensor”, to Ball et al, or U.S. Pat. No. 5,564,832, “Birefringent ActiveFiber Laser Sensor”, to Ball et al, which are incorporated herein byreference.

[0070] Referring to FIG. 19, another type of tunable fiber laser thatmay be used in an accelerometer 22 of the present invention is a tunabledistributed feedback (DFB) fiber laser, such as that described in V. C.Lauridsen, et al, “Design of DFB Fibre Lasers”, Electronic Letters, Oct.15, 1998, Vol.34, No. 21, pp 2028-2030; P. Varming, et al, “Erbium DopedFiber DGB Laser With Permanent π/2 Phase-Shift Induced by UVPost-Processing”, IOOC'95, Tech. Digest, Vol. 5, PD1-3, 1995; U.S. Pat.No. 5,771,251, “Optical Fibre Distributed Feedback Laser”, toKringlebotn et al; or U.S. Pat. No. 5,511,083, “Polarized Fiber LaserSource”, to D'Amato et al. In that case, a grating 316 is written in arare-earth doped fiber and configured to have a phase shift of λ/2(where λ is the lasing wavelength) at a predetermined location 318 nearthe center of the grating 316 which provides a well defined resonancecondition that may be continuously tuned in single longitudinal modeoperation without mode hopping, as is known. Alternatively, instead of asingle grating, the two gratings 310,312 may be placed close enough toform a cavity having a length of (N+½)λ, where N is an integer(including 0) and the gratings 310,312 are in rare-earth doped fiber.

[0071] Referring to FIG. 16, instead of positioning the gratings 310,312outside the wrap 302, they may be placed along the wrap 302. In thatcase the grating reflection wavelength may vary with accelerationchanges, and such variation may be desired for certain configurations(e.g., fiber lasers) or may be compensated for in the optical signalinstrumentation 35 (FIG. 1) for other configurations, e.g., by allowingfor a predetermined range in reflection wavelength shift for each pairof gratings.

[0072] Alternatively, instead of each of the wraps being connected inseries, they may be connected in parallel, e.g., by using opticalcouplers (not shown) prior to each of the wraps, each coupled to thecommon fiber 66.

[0073] Referring to FIG. 17, alternatively, the accelerometer 22 mayalso be formed as a purely interferometric sensor by wrapping themandrels (86, 88, 90, 92, of FIG. 4, and similar) with the wrap 302without using Bragg gratings where each wrap has a separate fiber 66. Inthis particular embodiment, known interferometric techniques may be usedto determine the length or change in length of the fiber 66 between themandrels due to movement of the mass 156 (FIG. 8), such as Mach Zehnderor Michaelson Interferometric techniques, such as that described in U.S.Pat. No. 5,218,197, entitled “Method and Apparatus for the Non-invasiveMeasurement of Pressure Inside Pipes Using a Fiber Optic InterferometerSensor” to Carroll. The interferometric wraps may be multiplexed such asis described in Dandridge, et al, “Fiber Optic Sensors for NavyApplications”, IEEE, February 1991, or Dandridge, et al, “Multiplexedinterferometric Fiber Sensor Arrays”, SPIE, Vol. 1586, 1991, pp 176-183.Other techniques to determine the change in fiber length may be used.Also, reference optical coils (not shown) may be used for certaininterferometric approaches and may also be located in or around theaccelerometer 22 but may be designed to be insensitive to axialaccelerations.

[0074] Also, for any geometry of the wraps described herein, more thanone layer of fiber may be used depending on the overall fiber lengthdesired. It is further within the scope of the present invention thewrap 302 may comprise the optical fiber 66 disposed in a helical pattern(not shown) about the mandrels. Other geometries for the wraps may beused if desired. The desired axial length of any particular wrap is setdepending on the characteristics of the ac sensitivity, and otherparameters, desired to be measured, for example the magnitude of theacceleration to be measured.

[0075] Referring to FIGS. 18 and 19, embodiments of the presentinvention include configurations wherein instead of using the wrap 302,the fiber 66 may be disposed on or within an elastic member 300 similarto those described herein above with reference to the various figures.In that case, the fiber may have shorter sections 314 that are disposedon the elastic support members that optically detect strain in themembers. The orientation of the strain sensing element will vary thesensitivity to strain on the member caused by acceleration.

[0076] Referring to FIGS. 20 and 21, alternatively the optical strainsensor 320, 322 on the support member 300 may have a longer length withvarious alternative geometries, such as a “radiator coil” geometry 320(FIG. 20) or a “race-track” geometry 322 (FIG. 21), which would bedisposed along the support member to measure strain. In this particularembodiment, the length will be set long enough to optically detect thechanges to the strain on the elastic member and the acceleration therebyas described herein above.

[0077] Referring to FIG. 18, in particular, the pairs of Bragg gratings(310,312), may be located along the fiber 66 with at least a section 314of the fiber 66 between each of the grating pairs located on the elasticmembers 300. In that case, known Fabry Perot, interferometric,time-of-flight or fiber laser sensing techniques may be used to measurethe change in length of at least a section of the elastic support member300, in a manner similar to that described in the aforementionedreferences.

[0078] Referring to FIG. 18, alternatively, the gratings 310, 312 may beindividually disposed on the support members 300 and used to sense thestrain on the members (and thus displacement of the mass 156). When asingle grating is used support member, the grating reflection wavelengthshift will be indicative of changes in strain on the member.

[0079] Any other technique or configuration for an optical strain gagemay be used. The type of optical strain gage technique and opticalsignal analysis approach is not critical to the present invention, andthe scope of the invention is not intended to be limited to anyparticular technique or approach.

[0080] For any of the embodiments described herein, the strain sensors,including electrical strain gages, optical fibers and/or gratings amongothers as described herein, may be attached to the elastic supportmembers by adhesive, glue, epoxy, tape or other suitable attachmentmeans to ensure suitable contact between the strain sensor and theelastic member. The strain gages, optical fibers or sensors mayalternatively be removable or permanently attached via known mechanicaltechniques such as mechanical fastener, spring loaded, clamped, clamshell arrangement, strapping or other equivalents. Alternatively, thestrain gages, including optical fibers and/or gratings, may be embeddedin the elastic members. Also, for any of the embodiments describedherein the support member may also comprise any strain sensitivematerial, such as a PVDF.

[0081] Referring to FIGS. 22, 23 it is also within the scope of thepresent invention that any other strain sensing technique may be used tomeasure the variations in strain on the elastic member, such as highlysensitive piezoelectric, electronic or electric, strain gages attachedto or embedded in the elastic support members. Referring to FIG. 22,different known configurations of highly sensitive piezoelectric straingages are shown and may comprise foil type gages 340. Referring to FIG.23, an embodiment of the present invention is shown wherein the strainsensors comprise strain gages 330. In this particular embodiment straingages 340 are disposed about a predetermined portion of the elasticmember 300.

[0082] It should be understood that any of the embodiments describedherein may comprise elastic support members in the form of discretestrips of material that are merely attached to the housing 158 and themass 156 by any known method. It should be further understood thatalthough description of the embodiments has been given with reference tothe mass 156 moving, it is within the scope of the present inventionthat the housing 15 8 may move and the mass remain stationary, therelative motion between the two features being detected by the change inlength of the support member.

[0083] It should be understood that, unless otherwise stated herein, anyof the features, characteristics, alternatives or modificationsdescribed regarding a particular embodiment herein may also be applied,used, or incorporated with any other embodiment described herein. Also,the drawings shown herein are not drawn to scale.

[0084] Although the invention has been described and illustrated withrespect to exemplary embodiments thereof, the foregoing and variousother additions and omissions may be made therein and thereto withoutdeparting from the spirit and scope of the present invention.

What is claimed is:
 1. A highly sensitive linear accelerometer forsensing acceleration in a predetermined direction, said accelerometercomprising: a rigid housing; a mass; at least two elastic supportmembers axially aligned in said predetermined direction and attached toopposite ends of said housing and further attached to said mass, saidelastic support members suspending said mass within said housing; and atleast a portion of one of said elastic support members comprising atransducer capable of measuring a displacement of said mass within saidhousing in response to an acceleration along said predetermineddirection.
 2. The accelerometer of claim 1 further comprising: a pair offixed mandrels rigidly attached to opposite ends of said housing; andwherein said mass comprises at least one floating mandrel and whereinsaid elastic support members comprise a wrap and wherein said elasticsupport members are disposed about one said fixed mandrel and saidfloating mandrel.
 3. The accelerometer of claim 2 wherein said masscomprises a pair of floating mandrels and wherein each said elasticsupport member is disposed about one of said fixed mandrels and one ofsaid floating mandrels.
 4. The accelerometer of claim 2 wherein saidmandrels and said mass comprise a toroidal shape.
 5. The accelerometerof claim 1 wherein at least one of said elastic support memberscomprises an optical fiber coil.
 6. The accelerometer of claim 5 whereinsaid and wherein said movement induces in said fiber a variation inlength corresponding to said movement for interferometric measurement ofsaid variation in length.
 7. The accelerometer of claim 1 furthercomprising an axial alignment assembly attached to said mass limitingmovement of said mass in a direction perpendicular to said predetermineddirection.
 8. The accelerometer of claim 7 wherein said axial alignmentassembly comprises a flexure member attached to said mass and saidhousing allowing axial movement of said mass in said predetermineddirection and limits non-axial movement of said mass.
 9. Theaccelerometer of claim 7 wherein said flexure member comprises adiaphragm.
 10. The accelerometer of claim 9 further comprising: a pairof said axial alignment assemblies further comprising an alignment rodwherein said diaphragm is disposed on an end of said rod; a borepositioned in each end of said housing; and wherein said diaphragms arecaptured within said bore about the periphery of said diaphragms. 11.The accelerometer of claim 9 further comprising: a pair of said axialalignment assemblies, wherein said axial alignment assemblies furthercomprise an alignment rod wherein said diaphragm is disposed on an endof said alignment rod; a bore positioned in each end of said fixedmandrels; and wherein said diaphragms are captured within said boreabout an outer periphery of said diaphragms.
 12. The accelerometer ofclaim 8 wherein said flexure member comprises a thin flexible platewherein at least one pair of said flexure members are attached to saidmass at a first end and to said housing at second end.
 13. Theaccelerometer of claim 1 wherein said transducer comprises a strainsensing element.
 14. The accelerometer of claim 13 wherein said strainsensing element comprises a fiber optic sensor, a piezo electric device,a PVDF material or a resistive strain gage.
 15. The accelerometer ofclaim 1 further comprising an outer housing having end caps wherein saidaccelerometer is disposed within said outer housing.
 16. A highlysensitive linear accelerometer for sensing acceleration in apredetermined direction, said accelerometer comprising: a rigid housing;a mass having an elongated body and a first and second rounded ends; afirst fixed mandrel and a second fixed mandrel, both rigidly attached tosaid housing a predetermined distance apart; a first pair of elasticsupport members axially aligned in said predetermined direction andwrapped around said first fixed mandrel and said first rounded end in acontinuous fashion; a second pair elastic support members axiallyaligned in said predetermined direction and wrapped around said secondfixed mandrel and said second rounded end in a continuous fashion; atleast a portion of one of said elastic support members comprising atransducer capable of measuring a displacement of said mass within saidhousing in response to an acceleration along said predetermineddirection; and a pair of axial alignment assemblies attached to saidmass limiting movement of said mass in a direction perpendicular to saidpredetermined direction.
 17. The accelerometer of claim 15 wherein saidfirst and second fixed mandrels and said mass are comprised of atoroidal shape.
 18. An apparatus for vertical seismic profiling of theearth having an x-direction, a y-direction and a z-direction orthogonalto each other, said apparatus comprising: an optical fiber transmissioncable; and a plurality of linear accelerometers coupled to the earth andin optical communication with said optical fiber transmission cable andpositioned in each of the three orthogonal directions, each said linearaccelerometer comprising highly sensitive accelerometer for sensingacceleration in a predetermined one of said directions, saidaccelerometer comprising: a rigid housing; a mass; and at least twoelastic support members comprised of optical fiber axially aligned insaid predetermined direction and attached to opposite ends of saidhousing and further attached to said mass, said elastic support memberssuspending said mass within said housing wherein at least a portion ofone of said elastic support members comprises a transducer capable ofmeasuring a displacement of said mass within said housing in response toan acceleration along said predetermined direction and providing arespective sensing light signal indicative of static and dynamic forcesat a respective accelerometer location.
 19. The apparatus of claim 18further comprising an optical signal processor connected to said opticaltransmission cable providing seismic profile information based on saidrespective sensing light signal.
 20. The apparatus of claim 18 furthercomprising an array of said linear accelerometers coupled to the earthat a plurality of predetermined positions.
 21. The apparatus of claim 18wherein the plurality of accelerometers are coupled to the earth via anoil well casing, a bore hole, or an oil production tube.